Once the photo is in your message window (you need to drag and drop it from another browser window), click on it and then click the small image icon along the top of the Message window (next to the smiley face).
An Image detail window should pop up (not all browsers support this) and you can enter any desired width into the "Width:" box (I suggest no larger than 650 px). Then click the OK button at bottom right and the edit window should close with the resized image in your message window.
Note that some browsers / operating systems may not support this feature. I've recently tested it on Firefox and Chrome (on Windows) and they both work.